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Optimization of NO oxidation by H2O2 thermal decomposition at moderate temperatures

  • Hai-qian Zhao ,

    Roles Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Software, Supervision, Visualization, Writing – original draft, Writing – review & editing

    dqzhaohaiqian@163.com

    Affiliation Department of Civil Engineering & Architecture, Northeast Petroleum University, Daqing, China

  • Zhong-hua Wang,

    Roles Formal analysis, Investigation, Resources, Validation, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Civil Engineering & Architecture, Northeast Petroleum University, Daqing, China

  • Xing-cun Gao,

    Roles Investigation, Software, Validation, Writing – review & editing

    Affiliation Department of Civil Engineering & Architecture, Northeast Petroleum University, Daqing, China

  • Cheng-hao Liu,

    Roles Investigation, Software, Validation, Writing – review & editing

    Affiliation Department of Civil Engineering & Architecture, Northeast Petroleum University, Daqing, China

  • Han-bing Qi

    Roles Methodology, Project administration, Resources, Supervision

    Affiliation Department of Civil Engineering & Architecture, Northeast Petroleum University, Daqing, China

Abstract

H2O2 was adopted to oxidize NO in simulated flue gas at 100–500°C. The effects of the H2O2 evaporation conditions, gas temperature, initial NO concentration, H2O2 concentration, and H2O2:NO molar ratio on the oxidation efficiency of NO were investigated. The reason for the narrow NO oxidation temperature range near 500°C was determined. The NO oxidation products were analyzed. The removal of NOx using NaOH solution at a moderate oxidation ratio was studied. It was proven that rapid evaporation of the H2O2 solution was critical to increase the NO oxidation efficiency and broaden the oxidation temperature range. the NO oxidation efficiency was above 50% at 300–500°C by contacting the outlet of the syringe needle and the stainless-steel gas pipe together to spread H2O2 solution into a thin film on the surface of the stainless-steel gas pipe, which greatly accelerated the evaporation of H2O2. The NO oxidation efficiency and the NO oxidation rate increased with increasing initial NO concentration. This method was more effective for the oxidation of NO at high concentrations. H2O2 solution with a concentration higher than 15% was more efficient in oxidizing NO. High temperatures decreased the influence of the H2O2 concentration on the NO oxidation efficiency. The oxidation efficiency of NO increased with an increase in the H2O2:NO molar ratio, but the ratio of H2O2 to oxidized NO decreased. Over 80% of the NO oxidation product was NO2, which indicated that the oxidation ratio of NO did not need to be very high. An 86.7% NO removal efficiency was obtained at an oxidation ratio of only 53.8% when combined with alkali absorption.

Introduction

SO2 and NOx are two major pollutants released by coal combustion. Since most NOx in coal-fired flue gas is NO, which is difficult to directly absorb with a liquid absorbent,[1] the removal of NOx in flue gas is more difficult than that of SO2. NOx has surpassed SO2 as the largest gaseous pollutant emission in recent years in China.[2] NOx emissions have reached twice that of SO2 in some Chinese cities.[2] Therefore, China revised the emission standards for pollutants from thermal power plants in 2011, in which the NOx emission standard was changed to 100 mg/Nm3 for most pulverized-coal boilers.[3] It is imperative to develop economical and efficient NOx removal technology.

At present, successfully commercialized denitrification technologies include low NOx burner technology (LNB), selective catalytic reduction technology (SCR) and selective non-catalytic reduction technology (SNCR). SCR can achieve high NOx removal efficiency; however, the initial investment and operational cost are expensive.[4] In addition, SCR has secondary pollutions, such as ammonia leak and waste catalyst. LNB can control the emission of NOx from a combustion source without greatly increasing the cost. In recent years, LNB technology has made great progress, and it can control the emission concentration of NOx to below 300 mg/Nm3 in the flue gas from boilers burning bituminous coal.[5]Therefore, it is a promising NOx control technology that should be widely developed.

Wet flue gas desulfurization (WFGD) units are commonly installed in thermal power plants. To realize the simultaneous removal of SO2 and NOx in present WFGD units based on LNB technology will be an economical way to meet the new NOx emission standard. However, most NOx in coal-fired flue gas is NO, which is difficult to absorb in SO2 scrubbers. Developing low-cost and high-efficiency NO oxidization technology is key to realizing the simultaneous removal of SO2 and NOx in WFGD units.

H2O2 is widely used as an oxidizing agent for its strong oxidation ability and environmental friendliness. Since the 1990s, researchers have injected H2O2 solution into flue gas to oxidize NO and gained many valuable results.[1,68] Zamansky and coworkers [6] found that the maximal conversion efficiency of NO can reach 90% at 500°C, when the molar ratio of H2O2:NO was 1.5. Kasper and coworkers[7] achieved a 97% oxidization efficiency of NO when the molar ratio of H2O2:NO was 2.6. Collins et al. [8] achieved high conversions of NO using molar ratios of H2O2:NOx slightly above 1.0. These studies have two similarities. First, the optimum temperatures were 500°C, and when the temperature deviated from 500°C, the NO oxidation efficiency dropped sharply. Second, NO was proven to be oxidized to NO2, HNO2, and HNO3; however, the proportion of various products was not determined. Thus, all researchers aimed for a high oxidization ratio of NO (the molar ratio of oxidized NO to total amount of NO in the flue gas).

For power plant boilers, it is difficult to find a position where the flue gas temperature is exactly 500°C because the flue gas temperature changes greatly over the course of flow. In addition, even in the same flue section, the flue gas temperature varies greatly. However, flue gas with temperature of 500°C can be found in economizers, but it is difficult to inject H2O2 solution at this site. The temperature of flue gas from the outlet of an economizer is approximately 400°C or lower. If the temperature range over which H2O2 efficiently oxidizes NO can be expanded, the possible application of this technology will be greatly increased.

The capacity for H2O2 to oxidize NO mainly comes from the free radicals generated from the decomposition of H2O2, such as hydroxyl radicals (·OH) and hydroperoxyl radicals (HO2·).[911] H2O2 thermal decomposition is one of the simplest means to generate ·OH. The bond length of HO-OH is 148 pm, and the bond dissociation energy is 213.8 kJ/mol.[12] H2O2 can be quickly decomposed below 500°C, even if it is not homogeneous. Lin et al. [13] studied the decomposition kinetics of H2O2 at 100–280°C. They found that the decomposition of H2O2 was a first-order reaction regardless of the temperature and reactor type, and the temperature and the reactor type had a great influence on the H2O2 decomposition rate. The results also showed that the H2O2 decomposition rate was fast at 280°C (more than 97% of the H2O2 decomposed in 50 s in Titanium tubing). Mok et al. [14] studied the decomposition characteristics of H2O2in propulsion system (on the surface of silver screen catalyst bed) at higher pressures. It was shown that the decomposition of H2O2 changed from a heterogeneous reaction to a homogeneous reaction at the temperature of 427°C. Even if the temperature was below 427°C, H2O2 can still decompose quickly (k = 10l3exp(-48000/RT) s-1).[15] Ali-zade[16] investigated the gas-phase oxidation reaction kinetics of pyridine derivatives with H2O2. The H2O2 concentration was 20–30%, and the temperature was 250–400°C.The results showed that H2O2 can decompose to generate free radicals efficiently at this temperature.

According to the homogeneous oxidation mechanism of NO by H2O2,[1] the NO oxidation product should mainly be NO2. Under the effect of thermal energy, hydrogen peroxide decomposes, and ·OH radicals are generated. Then, NO rapidly reacts with ·OH to produce HONO. However, the generated HONO reacts with ·OH, and the reaction rate constant k of this reaction is high (k = 6.24×10−12(T/298 K)e-0.57 cm3/molecule·s).[17] The products are NO2 and H2O. A reaction between H2O2 and ·OH (reaction 4) also occurs after reaction 1.[18] HO2· generated from reaction 4 can oxidize NO to NO2.[19] It can be seen that the final product of the reaction between NO and ·OH or HO2· should be NO2. Thomas and Vanderschuren[20] studied nitrogen oxide scrubbing with alkaline solutions and found that the total dissolved NOx concentration reaches a maximum value for intermediate oxidation ratios (50%-70%). This indicates that if NO can be mainly oxidized to NO2, the oxidation ratio does not need to be very high, and 50% may be an ideal oxidation ratio for NOx absorption in WFGD units.

(1)(2)(3)(4)(5)

In this research, the NO oxidation efficiency under different H2O2 evaporation conditions was studied. On this basis, influences of the gas temperature (100–500°C), initial NO concentration, H2O2 concentration, and H2O2:NO molar ratio on the oxidation efficiency of NO were investigated in a drop-tube furnace. The NO oxidation products were analyzed. The removal of NOx using sodium hydroxide solution was studied at an oxidation ratio of 53.8%.

Experimental

NO oxidation by H2O2 thermal decomposition was performed in a drop-tube furnace NO oxidization experimental system, as shown in Fig 1. The reaction system is composed of a drop-tube furnace, a quartz oxidation reactor, a micro-membrane pump, a H2O2 solution container, a gas analyzer, a condenser and a gas distribution system. The length of the oxidation reactor was 680 mm, and the inner diameter was 50 mm.

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Fig 1. Drop-tube furnace NO oxidization experimental system.

1-NO; 2-N2; 3-gas mass flow meter; 4-buffer bottle (D = 100mm. H = 150mm); 5-drop-tube furnace; 6-stainless-steel gas inlet;7-syringe needle;8-micro-membrane pump; 9-H2O2 solution container; 10-gas analyzer; 11-condenser; 12-quartz oxidation reactor.

https://doi.org/10.1371/journal.pone.0192324.g001

The temperature inside the drop-tube furnace was kept at a constant value. N2+NO gas with different NO concentrations was provided by an N2 gas cylinder and an NO gas cylinder via gas mass flow meters at room temperature (25°C). The total gas flow was 1.5 L/min. N2+NO gas was delivered into the reactor through the stainless-steel pipe at the top of the quartz reactor. The pressure inner the reactor was 0.2MPa. H2O2 solution with a desired concentration was prepared and pumped into the reactor through a long syringe needle using a micro-syringe pump (LSP01-1A, Longer, China). In most experiments, the initial concentrations of H2O2 were 30%, and the H2O2:NO molar ratio was 10:1. The injected H2O2 solution vaporized and decomposed immediately in the oxidation reactor. Then, NO was oxidized by the free radicals formed from the decomposition of H2O2. To speed the evaporation rate of H2O2 droplets, the outlet of the syringe needle and the stainless-steel gas pipe were connected (Fig 2). This set-up rapidly converted the H2O2 droplet into a thin film on the surface of the stainless-steel gas pipe, which prevented H2O2 droplets from dripping directly to the bottom of the furnace without evaporation.

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Fig 2. Connection method of the syringe needle and the stainless-steel gas pipe.

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The pH value of H2O2 solution bought from Aladdin Industrial Corporation was adjusted to be about 5, and certain stabilizer was added. Under such conditions, H2O2 was stable at room temperature without catalysts. The H2O2 solution container was made of glass and the syringe was made of plastic. The connecting tubes in these experiments were mainly silicone tubes. So, the decomposition of H2O2in the tubes and in the syringe was negligible.

Before the experiment, the NO and N2 valves were opened, and N2+NO gas flowed into the oxidation reactor. The NO concentration was recorded by the gas analyzer (VARIO PLUS, MRU, Germany). The gas analyzer worked based on the electrochemical principle. NO gas was measured with 3-electrodes sensors.The gas valve opening was adjusted so that the total gas volume was 1.5 L/min and the initial concentration of NO reached 300 mg/m3. When the NO concentration was stable, the initial NO gas concentration was recorded. Then, H2O2 solution was injected into the reactor. H2O2 was heated and decomposed, and NO was oxidized. The outlet concentration of NO was monitored by the gas analyzer. The oxidation efficiency of NO could be calculated by the NO concentrations. where η represents the oxidation efficiency of NO, %; C0 represents the initial concentration of NO, mg/m3; and C1 represents the average concentration of NO in the reaction process, mg/m3.

Results

Effect of evaporation of H2O2solution on the gas temperature

A drop-tube furnace was used to maintain the reactor temperature at a constant value in these experiments, but the H2O2 solution was quickly evaporated, and it absorbed much of the latent heat of vaporization in a short time. This may have an effect on the gas temperature. The influence of H2O2 solution evaporation on the vertical temperature field in the reactor was studied. The temperatures of three different heights in the reactor were measured by a thermocouple. The positions of the measuring points are shown in Fig 3. The height of measuring point T1 was the same as that of the stainless-steel gas inlet. The measuring point T2 was 50 mm lower than the gas inlet. The height of the measuring point T3 was 50 mm lower than that of T2. The temperature of the reactor was 400°C. The flow of H2O2 solution was 0.08 mL/min, which was the maximum flow in this study. To ensure that the H2O2 solution was fully evaporated, the H2O2 syringe needle was connected to the stainless-steel gas inlet. The temperature test results at different measuring points are shown in Table 1.

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Fig 3. Temperature measuring point positions in the reactor.

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Table 1. Temperature test results at different measuring points.

https://doi.org/10.1371/journal.pone.0192324.t001

The results showed that the temperatures at the three measuring points were approximately 400 °C before the H2O2 solution was injected. As the measuring point T1 was located at the end of the furnace, the temperature was slightly lower than 400 °C (398.6 °C). This indicated that the internal temperature in the reactor was uniform. When the H2O2 solution was injected, the temperature at the T1 measuring point decreased significantly (382.5 °C). This was because measuring point T1 was located near the inlet of the H2O2 solution and the gas, where the H2O2 solution was injected and evaporated rapidly. Evaporation of the H2O2 solution quickly absorbed a large amount of heat, resulting in a large decrease in the temperature near the T1 measuring point. However, the temperature away from T1 quickly reached 400 °C. The temperature at T2 and T3 was close to 400 °C. It can be seen that the evaporation of H2O2 solution only affected the gas temperature near the gas inlet, and the temperature of the whole reactor was not affected.

Since the needle diameter was too small, it was difficult to measure its surface temperature. However, the height of the measuring point T1 was the same as that of the needle outlet, so the needle outlet temperature should be close to 382.5 °C after H2O2 solution was injected.

Effect of H2O2 solution evaporation

The evaporation of H2O2 solution had an important effect on the thermal decomposition of H2O2. If the H2O2 solution did not evaporate quickly, the H2O2 solution would form large droplets at the needle of the syringe. When the droplets grow large enough, they would drip rapidly, resulting in a very short residence time of H2O2 in the oxidation reactor. In this case, H2O2 could not be decomposed effectively. Only when the H2O2 solution was fully evaporated could H2O2 decompose and oxidize NO gas quickly. Therefore, NO concentrations were compared under different evaporation conditions. In experiment 1, the gas inlet and needle were not in direct contact, and H2O2 solution droplets dripped quickly through the reactor. In experiment 2, the H2O2 syringe needle was connected to the stainless-steel gas inlet, which promoted the rapid decomposition of H2O2. Fig 4 shows the effect of H2O2 solution evaporation on NO oxidization.

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Fig 4. Influence of H2O2 evaporation on NO oxidation.

The temperature in the reactor was 400°C.

https://doi.org/10.1371/journal.pone.0192324.g004

In experiment 1, the H2O2 solution did not evaporate rapidly. Most of the H2O2 was discharged from the bottom of the reactor in liquid form. Although the NO concentration decreased after the injection of H2O2, the NO concentration in the whole process did not remain stable, which reduced the average oxidation efficiency of NO. When the droplets evaporate well (in experiment 2), the concentration of NO gas decreased sharply and remained constant after the H2O2 solution was injected into the reactor. This indicated that the thermal decomposition of H2O2 was stable and that the decomposition products could oxidize NO efficiently. The results proved that sufficient evaporation of H2O2 solution was a prerequisite for high oxidation efficiency of NO. In the latter experiments, the syringe needle was connected to the stainless-steel gas inlet tube to ensure that the H2O2 solution was fully evaporated.

Collins et al. [8] injected a H2O2 solution into hot flue gas to control NOx emission. They found that the position of the injector and the type of atomization were very important to the efficient utilization of H2O2. This was most likely due to the position of the injector and the type of atomization affecting H2O2 evaporation and decomposition. The gas temperature and flow rate were not uniform in the boiler’s gas flue, and thus the evaporation rate of H2O2 in different locations was very different. If the injector position was not appropriate, H2O2 could not decompose quickly, and much of the H2O2 did not oxidize NO well, which led to the low oxidation efficiency of NO and low utilization of H2O2. The size of the H2O2 droplets ejected by different atomization methods was different. The smaller the droplet size, the larger the gas-liquid surface area under the same injection flow, which was beneficial to improve the evaporation rate of H2O2. Under a certain flue gas temperature, converting hydrogen peroxide solution to small droplets or thin liquid films would greatly improve the rate of evaporation and decomposition, thus ensuring high NO oxidation efficiency.

Effect of gas temperature

Temperature affects not only the evaporation and the decomposition of the H2O2 solution but also the rate of the NO oxidation reaction. The effect of gas temperature on NO oxidation was studied in this research. The results are shown in Figs 5 and 6. It can be seen from Fig 5 that the NO oxidation efficiency rose with an increase in the gas temperature between 100–500°C. At 100°C, H2O2 did not oxidize NO gas well. This was because the boiling point of pure H2O2 is approximately 150.2°C at atmospheric pressure, and the boiling point of H2O2 solution is higher than 100°C.[21] Therefore, the H2O2 solution could not evaporate efficiently at this temperature. This result also indicated that H2O2 did not oxidize NO directly, and the oxidation capacity of H2O2 to NO was mainly due to the free radicals produced from its decomposition. Compared to that at 100°C, the NO oxidation efficiency rose significantly at 200°C. Because 200°C is higher than the evaporation temperature of 30% H2O2 solution, the H2O2 solution could evaporate and decompose in order to oxidize NO. However, because the gas temperature was not very high, the solution evaporation rate was not fast enough. Liquid droplets could be seen at the bottom of the oxidation reactor. This illustrated that only a portion of the injected H2O2 evaporated and decomposed. From Fig 6, it can be seen that the NO concentration at 200°C varied greatly, which is similar to the result of experiment 1 (Fig 4) in which H2O2 did not evaporate quickly.

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Fig 5. Influence of gas temperature on NO oxidation.

The temperature in the reactor was between 100–500°C.

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Fig 6. NO concentration at different temperatures.

The temperature in the reactor was between 100–500°C.

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In the temperature range of 300–500°C, the NO oxidation efficiency was above 50%, and it changed slowly with an increase in the gas temperature. The maximum oxidation efficiency was obtained at 400°C. It was difficult to see droplets dripping to the bottom of the reactor when the temperature was higher than 300°C. The NO concentration had only small fluctuations at 300°C. The NO concentration remained low during the oxidation process so that the average oxidation efficiency of NO was high at temperatures over 300°C.

In the literature, the NO oxidation rate was reported to be high over a narrow temperature range near500 °C, and it decreased sharply when the temperature deviated (no matter increased or decreased) from 500 °C.[6] There are three reasons which cause this result. Firstly, when the temperature was too low, the H2O2 evaporation rate was too slow. Secondly, the reaction rate of the reaction between ·OH and NO decreased with the increase in temperature.[22] If the temperature was too high, NO oxidation rate would decline. Thirdly, the wall destruction of the free radicals was accelerated when the gas temperature was high. This result is very unfavourable to the implementation of this technology. Because the gas temperature is uneven the flue gas flows. Even in the same cross section, the flue gas temperature is quite different. When H2O2 is injected into a flue gas, the NO oxidation efficiency will be low because of the uneven temperature field. It is important to ensure that the NO can be efficiently oxidized over a wide temperature range.

The results in this research were quite different from that in the literature. High NO oxidation efficiency could be obtained from 300 °C to 500 °C when the H2O2 solution evaporated and decomposed rapidly. The reason was that the evaporation rate of injected H2O2 was accelerated by converting it into very thin liquid films on the surface of the stainless-steel gas pipe. High NO oxidation efficiency was obtained at 300 °C when the H2O2 evaporation rate was fast. This result indicated that it was possible to oxidize NO gas efficiently by injecting H2O2 solution into the real flue gas. There may be two main ways to accelerate the evaporation of the H2O2 solution. First, the droplet diameter can be reduced using advanced liquid atomization technology, such as ultrasonic atomizers. Second, adequate solid surfaces with high surface energy can be placed near the H2O2 solution injectors, on which the H2O2 solution could spread into very thin liquid films. As to the same liquid, the larger the solid surface energy, the smaller the contact angle of the liquid on the surface, and the easier it is to spread the liquid into the film.

Effect of the initial concentration of NO

There are significant differences in the NOx concentration in flue gas from different types of coal-fired boilers. When the boilers use low nitrogen combustion technology, the NOx concentration in the outlet flue gas is much lower. The influence of the initial NO concentration on the NO oxidation efficiency was studied in this experiment. As the concentration of the H2O2 solution did not change, the injection amount of the H2O2 solution was adjusted to maintain H2O2:NO = 10. The effect of the initial NOx concentration on the NOx oxidation efficiency is shown in Fig 7.

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Fig 7. Influence of the initial NO concentration on NO oxidation.

The temperature in the reactor was 400°C. The initial NO concentration ranged from 100–500 mg/m3.

https://doi.org/10.1371/journal.pone.0192324.g007

As shown in Fig 7, the NO oxidation efficiency increased monotonically with an increase in the initial NO concentration. The NO oxidation rate increased linearly with the increase in the initial NO concentration. This was because the higher the initial NO concentration, the faster the reaction rate between NO and ·OH radicals or HO2· radicals. As the gas flow rate was constant, the time that the N2+NO gas remained in the reactor did not change. The oxidation rate and oxidation efficiency of NO increased with an increase in the NO concentration over the same reaction time. It can be seen that this method was more effective for the oxidation of NO at high concentrations.

Effect of H2O2 concentration

Under the same H2O2:NO molar ratio, the higher the H2O2 concentration, the less amount of H2O2 solution that needs to be injected. Water vapor absorbs less of the latent heat of vaporization, which has less impact on the gas temperature. When the injected H2O2 concentration is too low, more H2O2 solution is required to oxidize the same amount of NO. The flue gas temperature may decline significantly, and this will decrease the NO oxidation efficiency. Therefore, it is necessary to optimize NO oxidation using different concentrations of H2O2. The effect of the H2O2 solution concentration on the NO oxidation efficiency is shown in Fig 8.

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Fig 8. Effect of H2O2 concentration on NO oxidation efficiency.

The concentration of the H2O2 solution was between 2.5%-30%. The temperature in the reactor was 300–500°C.

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Fig 8 shows that the higher the concentration of H2O2 solution under the same temperature conditions, the higher the NO oxidation efficiency. When the concentration was less than 15%, the NO oxidation efficiency increased with an increase in the H2O2 solution concentration. However, when the H2O2 concentration was higher than 15%, the NO oxidation efficiency increased only slightly. Because the H2O2:NO molar ratio remained constant, the volume of H2O2 solution injected into the gas could be decreased when using a high concentration of H2O2 solution. This would reduce the heat absorbed by the evaporation of water. When the concentration of the H2O2 solution was less than 15%, the large volume of H2O2 solution injected into the reactor greatly influenced the gas temperature, and the evaporation rate of H2O2 solution was greatly reduced, which decreased the oxidation efficiency of NO. It can also be seen from Fig 8 that the higher the gas temperature, the smaller effect of the H2O2 solution concentration on the NO oxidation efficiency. Considering the results of different gas temperature conditions, it is suggested that the H2O2 concentration is not lower than 15%.

In the previous studies, H2O2 solution with the concentration of 50% was used to oxidize NO.[68] The reason was that low concentrations of H2O2 seriously affected the solution evaporation rate, which decreased the NO oxidation efficiency. In this study, H2O2 evaporation rate was accelerated by spreading H2O2 solution into a thin film on the surface of the stainless-steel gas pipe. Under this condition, H2O2 concentration was decreased to 15%. The result illustrated that H2O2 solution with low concentration can be used to oxidize NO, as long as the evaporation rate is fast enough.

Effect of H2O2:NO molar ratio

The H2O2:NO molar ratio directly determines whether the H2O2 injection method is economical compared to SCR technology. Haywood et al. proposed that at a molar ratio of 1.37:1, the method was an economically feasible alternative to the SCR method for NOx control.[4] If the NO in flue gas is pre-oxidized by injecting H2O2 and the oxidized N species (NO2 and HNOx) are removed along with SO2 by ammonia, the compound fertilizer of ammonium nitrate and ammonium sulfate will be produced. This compound fertilizer is expensive. Considering this by-product, the H2O2:NO molar ratio can be increased. However, the exact molar ratio needs to be recalculated. In this study, the NO oxidation efficiency was studied under different H2O2:NO mixture ratios. The results are shown in Fig 9.

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Fig 9. Influence of H2O2:NO molar ratio on NO oxidation.

The H2O2:NO molar ratios were 1:1, 2.5:1, 5:1, 10:1 and 50:1. The temperature in the reactor was 500°C.

https://doi.org/10.1371/journal.pone.0192324.g009

Fig 9 shows that when H2O2:NO = 1, the oxidation efficiency of NO was 31.4%. The oxidation efficiency of NO increased with the an increase in the H2O2:NO molar ratio. After the H2O2:NO molar ratio reached 10, the NO oxidation efficiency did not increase with an increase in the molar ratio. Although the NO oxidation efficiency increased with the H2O2:NO molar ratio, the ratio of H2O2 to oxidized NO declined. This illustrated that the H2O2 utilization efficiency decreased with an increase in the molar ratio. Therefore, to reduce operation costs, it is crucial to obtain an appropriate NO oxidation efficiency by reducing the H2O2:NO molar ratio.

The result also proved the result in Fig 7, in which NO oxidation efficiency was only affected by NO concentration, but not H2O2 concentration. It can be seen from Fig 9 that when the molar ratio exceeded 10, the NO oxidation efficiency increased slowly. Under these conditions, the concentration of H2O2 increased with the increase of molar ratio, but it did not significantly promote NO oxidation efficiency. The results in Fig 7 were obtained at H2O2:NO = 10. When NO concentration increased from 300 mg/m3 to 500 mg/m3, H2O2 concentration also increased by 5/3 times to maintain H2O2:NO ratio.As a result, NO oxidation efficiency increased by 6.3% in Fig 7. However, if NO concentration maintained 300 mg/m3, H2O2 concentration still increased by 5/3 times, NO oxidation efficiency did not change significantly according to the result in Fig 9. Therefore, the increase of NO oxidation efficiency in Fig 7 must be the result of the effect of NO concentration.

NO oxidation products

The optimal oxidation ratio of NO was determined by the NO conversion products, as the dissolution characteristics of the various products are different. Limvoranusorn et al. [1] reported that the main products of NO oxidation by H2O2 thermal decomposition were NO2, HNO2 and HNO3. To obtain more soluble HNOx, a high NO oxidation ratio was pursued in their study. If the oxidation product is mainly NO2, a high NO oxidation ratio may not be conducive to the absorption of NOx because NO will be regenerated when NO2dissolves in water (reaction 6).[23,24] Some scholars have found that when the NO oxidation ratio was 50%-70%, the absorption rate of NO gas in alkaline solution was the highest.[20] Because equimolar amounts of NO and NO2aid in their dissolution (reaction 7),[23,24] it is very important to detect the products of NO oxidation by the H2O2 injection method to determine the reasonable oxidation ratio. The test results of the NOx concentration at the reactor outlet after injection of the H2O2 solution are shown in Fig 10.

(6)(7)
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Fig 10. The change in NO2 concentration.

The temperature in the reactor was 400°C.

https://doi.org/10.1371/journal.pone.0192324.g010

After the injection of H2O2, the NO concentration decreased significantly, and the NO2 concentration increased along with the reduction of NO. The NO concentration dropped from 300 mg/m3 to approximately 140 mg/m3, and the average NO reduction was 162.9 mg/m3. The NO2 concentration increased from 0 to approximately 130 mg/m3, and the average NO2 concentration was 131.4 mg/m3. The reduction in the NO concentration was only 31.5 mg/m3 higher than the NO2 concentration, which indicated that over 80% of the NO oxidation product was NO2. It can be deduced that reactions 1–5 were the primary reactions in the system. The reactions began with the decomposition of H2O2 (reaction 1), and then some of the ·OH and H2O2 reacted to generate HO2· radicals. Some of the ·OH radicals reacted with NO to form HNO2, and the generated HNO2 converted to NO2 quickly through reaction 3, followed by reaction 2. Reaction 8 was not the main reaction due to its slow reaction rate.[25] (8) (9) (10) (11) (12) (13)

NOx absorption by NaOH solution

In this experiment, NaOH solution was used to absorb NOx from the gas in the oxidation reactor. The NaOH solution volume was 1.0 L, and the solution was placed in a bubbling reactor. The inner diameter of the bubbling reactor was 100 mm, and the volume was 1.09 L. The volume of gas in the bubbling reactor was small and thus had little effect on the concentration of NOx when the gas was bubbled into the reactor. After the hydrogen peroxide solution was injected into the oxidation reactor, the gas first entered the gas analyzer to record the NOx concentration change. When the NO and NO2 concentrations were stabilized, the gas from the oxidation reactor was directed into the NaOH solution. Then, the NO and NO2 concentrations were measured using the gas analyzer. The results of the NOx concentration measurements are shown in Fig 11.

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Fig 11. NOx removal by NaOH solution.

The temperature in the reactor was 400°C. The concentration of the NaOH solution was 2 mol/L.

https://doi.org/10.1371/journal.pone.0192324.g011

In this experiment, the oxidation efficiency of NO was 53.8%, and the concentration of NO2 was slightly less than that of NO. It can be seen that the NaOH solution was very effective in absorbing NOx at this oxidation ratio. The average concentration of NO2 at the outlet of the bubbling reactor was 16.7 mg/m3, and the average concentration of NO was 23.3 mg/m3. The total NOx concentration was 40.0 mg/m3. The NOx removal efficiency was 86.7%. It can be seen that NO oxidation by H2O2 thermal decomposition is an ideal pre-oxidation technology, which when combined with alkali absorption, can achieve a high NO removal efficiency.

Due to the size of the oxidation reactor, the maximum oxidation efficiency of NO was approximately 54% at a NO concentration of 300 mg/m3, which was lower than that in the experiment of Collins and coworkers. However, they found that the NO oxidation efficiency in the pilot-scale experiment was much higher than that in the laboratory studies. If this technology is used to remove NO from real power plant flue gas, the H2O2:NO molar ratio would be significantly reduced, and the NO oxidation efficiency would be greatly enhanced. It is easy to obtain an NO oxidation efficiency of 50% at a very low H2O2:NO molar ratio. The required NOx emission concentration is less than 100 mg/m3 in China's new thermal power plant emission standards issued in 2011. This means that the NOx emission concentration from flue gases from most existing pulverized coal boilers would meet the new emission standards when using this method. The method may be an economic alternative to SCR technology.

This study confirmed that the oxidation efficiency did not need to be too high, and good removal could be obtained at an oxidation ratio of 50% when combined with alkali absorption. The temperature range for NO oxidation could be widened to 300–500 °C by the enhancement in the H2O2 evaporation rate. The injection position is conveniently behind the economizer, which makes the technology easy to implement. This method will be an economically feasible NO pre-oxidation technology for the simultaneous removal of SO2 and NOx using WFGD units.

Discussion

In this study, H2O2:NO molar ratio was 10 in most experiments. Under these conditions, the generation rate of O2 was so low that it was difficult to detect. Only when H2O2:NO molar ratio was high (such as 50), a small amount of O2 (concentration below 0.3%) can be detected. There are two main reasons for the low O2 concentration. Firstly, the injection amount of H2O2 was relatively small, resulting in low O2 concentration. The initial concentration of NO was 300 mg/m3, and the total gas flow rate was 1.5 L/min. It can be calculated that the flow rate of NO was only 10−5 mol/min. When H2O2:NO = 50, the injection rate of H2O2 was 5×10−4 mol/min. The maximum generation rate of O2 was 2.5×10−4 mol/min (2H2O2→O2). The concentration of O2 in the flow was 2.5×10−4×22.4/1.5×100% = 0.37%, which was very low. Secondly, not all free radicals generated from the decomposition of H2O2 were involved in O2 generation reactions. Many free radicals reacted with NO, H2O2 or the mid products (reaction 2–5), which decreased the yield of O2.

In addition, reaction 9 is a third order reaction. The reaction rate constant is only 3.30×10−39×e-4.41/RT cm6/molecule2 s, which is much lower than reaction 2 (3.30×10−11 cm3/molecule s).[22]

We have tried the NO oxidation by 0.3% O2 at 500 °C, however, the concentration of NO did not changed. Based on the above analysis, we believe that the oxidation of NO by O2 is negligible.

In addition, when the NO initial concentration was 300mg/m3, the maximum NO oxidation efficiency was less than 55%. We found that the maximum oxidation efficiency was limited by the size of the reactor. We have tried two reactors with different size. In this study, the larger reactor was used. The length of the larger reactor was 680 mm and the inner diameter was 50 mm. The flow rate was 1.5 L/min, so the residence time was 54 s. The smaller reactor was 500 mm long and 38 mm in diameter. The flow rate was 0.65 L/min, and the residence time was 52 s. Due to the high rate of the reaction between NO and ·OH, the residence time was adequate. The other parameters such as temperature, initial concentration of NO, H2O2:NO were the same. Though the residence time in two reactors was almost the same, the maximum NO oxidation efficiency had a great difference. The maximum NO oxidation efficiency in the larger reactor was near 55%, but the maximum NO oxidation efficiency in the smaller reactor was only 43%. The higher oxidation efficiency in the larger reactor was not due to the higher injection amount of H2O2. Because the concentrations of H2O2 in the gases were the same, we believe that the free radicals have a wall destruction effect. The wall in the large reactor is far from the reaction zone, which weakened the useless destruction of the free radicals.

Conclusions

The general conditions of NO oxidation by H2O2 thermal decomposition were investigated in the drop-tube furnace NO oxidization experimental system. The conclusions are as follows:

  1. Sufficient evaporation of the H2O2 solution was a prerequisite to achieve the high oxidation efficiency of NO. The NO oxidation efficiency could be obtained over a wide temperature range when the H2O2 solution evaporated and decomposed rapidly. The NO oxidation efficiency was above 50% in the temperature range of 300–500°C in this research.
  2. The NO oxidation efficiency and the NO oxidation rate increased with an increase in theinitial NO concentration. This method was more effective for the oxidation of NO at high concentrations.
  3. High temperatures decreased the influence of the H2O2 concentration on the NO oxidation efficiency. The higher the concentration of H2O2 solution under the same temperature conditions, the higher the NO oxidation efficiency. Considering the results of different gas temperature conditions, it is suggested that the H2O2 concentration is not lower than 15%.
  4. The oxidation efficiency of NO increased with an increase in the H2O2:NO molar ratio, but the ratio of H2O2 to oxidized NO decreased. Oxidizing NO at an appropriate ratio with a low H2O2:NO molar ratio could reduce the cost of NO oxidation.
  5. It was proven that the NO oxidation product was mainly NO2 and that the oxidation ratio of NO did not need to be very high. An 86.7% NO removal efficiency was obtained at an oxidation ratio of 53.8% when combined with NaOH solution absorption.

Acknowledgments

This work is supported by National Natural Science Foundation of China (No. 51606036), Natural Science Foundation of Heilongjiang Province (No. QC2014C047) and Heilongjiang Provincial Department of Education Project (No. 12541093).

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